Imaging radar methods operate with an active radar sensor which, by means of radiating and receiving electromagnetic waves in the microwave range, generates a reflectivity map of the illuminated area. In recent years synthetic aperture radar (SAR) has attained major significance in remote sensing due to the high resolution and rich information content of SAR images. In addition to traditional applications in geography and in topographical and thematic mapping, SAR sensors also find application nowadays in many other fields such as e.g. in oceanography, agriculture and forestry, urban planning, ecology as well as in the forecasting and evaluating natural disasters.
One salient property of SAR methods materializes from the propagation properties of microwaves. Due to their long wavelength they are able to penetrate vegetation and even the ground down to a certain depth, depending on the wavelength as well as on the dielectric constant and density of the object concerned. Radiation of shorter wavelength, such as X band radiation, exhibits a strong attenuation and is backscattered primarily by the upper portions of the vegetation, whereas radiation of longer wavelength, such as L and P band typically penetrates deeper into the vegetation cover and ground. Backscatter thus contains contributions of all layers attained by the radiation.
One main problem in analyzing conventional SAR images of longer wavelengths is the layover of several backscatter contributions. Although a certain backscatter contribution of interest is buried in the data, this is often inaccessible since it is only the backscatter as a whole that can be sensed. Another problem is establishing the precise vertical position of the backscatter location, which is unknown since SAR geometry as a whole exhibits symmetry in elevation. This is why the elevation angle or the topographical elevation of the backscatter cannot be resolved by a conventional SAR method.
One known imaging SAR method is the so-called SAR interferometry (INSAR) as described in the paper R. Bamler and P. Hartl: “Synthetic Aperture Radar Interferometry”, Inverse Problems, Volume. 14, pages R1-R54, 1998, which is understood to be a technique by which the phase difference between two SAR images, taken at slightly different positions, is evaluated. This phase difference is a function of the elevation angle involved and thus of the topography of the terrain concerned. This permits generating highly accurate digital elevation models (DEMs) from INSAR images in which the elevation of a mean scatter center is decisive for each pixel of the image.
Making use of various wavelengths or polarizations opens up the additional possibility of determining the elevation of a plurality of scatter centers characteristic for each wavelength or polarization concerned; this can be used in determining the thickness of vegetation layers. By inverting simple scatter models, further physical parameters, such as e.g. the attenuation constant, can be additionally determined.
SAR interferometry (INSAR) has, however, more particularly the disadvantage that it is exclusively the elevation of a mean scatter center that can be measured, i.e. this method fails to achieve true three-dimensional imaging. Thus where volume targets are concerned, layover of the various scatter centers continues to be a problem. This is why the INSAR method is unsuitable for a detailed analysis of volume targets. On top of this, INSAR measurements fail to be completely unambiguous, and a further processing step is needed, namely phase unwrapping, to eliminate these ambiguities.
Model-based approaches on the basis of polarimetric or multifrequent interferograms are restricted, in principle, to a few simple parameters in analyzing volume targets. Their scope of application is thus very limited. Apart from this, they greatly depend on the scatter model employed and are a total failure when the assumptions made fail to apply.
Another technique for analyzing three-dimensional objects is multi-pass tomography, as known from the paper by A. Reigber and A. Moreira: “First Demonstration of Airborne SAR Tomography using Multibaseline L-band Data”, IEEE Trans. on Geoscience and Remote Sensing, Volume 38, No. 5, pages 2142-2152, September 2000. In this technique, a true three-dimensional image is obtained by coherent combination of a large number of SAR images at various viewing angles. The backscatter contributions of volume targets are separated in the elevation and can thus be analyzed each independently of the other. It is also possible to combine this technique with polarimetry to thus permit attaining indications not only as to the three-dimensional distribution of the scattering processes but also as to the type of the scattering process concerned in each case.
The main problem in multi-pass tomography is the extremely high experimental complications involved. Only with a large number of parallel passes (>10) good resolution coupled with a good information content, is attainable. This necessitates lengthy flight times for a relatively small imaged portion. Since the relative distances between the passes need to be known to within a millimeter for data processing, the multi-pass technique makes high demands on the positioning of the sensor. These requirements have hitherto been satisfied only to an inadequate extent. In conclusion, the unavoidable lack of uniformity in the distribution of the passes greatly restricts the quality of imaging. This is why multi-pass tomography is to be appreciated only as functional verification of airborne SAR tomography.
Known further from U.S. Pat. No. 5,463,397 A is a SAR interferometry system as a combination of multi-pass interferometry with successive dual-antenna SAR interferometry to obtain elevation maps with an accuracy unobtainable by either method alone. However, here too, the disadvantages of the multi-pass tomography as described above occur.
Separating the backscatter contributions in accordance with the elevation can also be achieved by the radar pulses emitted vertically downwards, as described in the paper by M. Hallikainen, J. Hyyppa, J. Haapanen, T. Tares, P. Ahola, J. Pulliainen, M. Toikka: “A helicopter-borne eight-channel ranging scatterometer for remote sensing—Part 1: System Description”, IEEE Trans. On Geoscience and Remote Sensing, Volume. 31, No. 1, pages 161-169, 1993. Here, unlike in the two methods as described above, resolving elevation is achieved by delay measurement of the radar.
Although a good elevation resolution is achievable by downwards emitted radar pulses, the three-dimensional resolution is poor. The swath of such an image needs to remain narrow to permit a near vertical angle of incidence, thus making large-area imaging impossible.
It is very similar to this that light detection and ranging (Lidar) sensors offer the basic possibility of three-dimensional object analysis. Here, instead of microwave pulses, short laser pulses are emitted vertically downwards, precise delay measurement in turn making elevation resolution possible. Thus, both methods enable profiles along the pass of the sensor to be determined. Similar as for radar pulses emitted vertically downwards, Lidar systems too, permit achieving only a narrow swath, since otherwise a fringing angle of incidence would not permit penetration of the laser pulses down to the ground. However, even with vertical incidence a Lidar system depends on small clearances in a forest in being able to penetrate to the ground and is thus greatly dependent on the type of forest concerned. Clouding too, prevents application of the Lidar system. In general, Lidar is only suitable for measuring the height of vegetation; it cannot be used for three-dimensional volume analysis.